Accuracy improvements:
1. We assumed the output was a p2wpkh, but it can be user-supplied now.
2. We assumed we always had change; remove this for wallet_select_all.
Calculation out-by-one fixes:
1. We need to add 1 byte (4 sipa) for the input count.
2. We need to add 1 byte (4 sipa) for the output count.
3. We need to add 1 byte (4 sipa) for the output script length for each output.
4. We need to add 1 byte (4 sipa) for the input script length for each input.
5. We need to add 1 byte (4 sipa) for the PUSH optcode for each P2SH input.
The results are now a slight overestimate (due to guessing 73 bytes
for signature, whereas they're 71 or 72 in practice).
Fixes: #458
Reported-by: Jonas Nick @jonasnick
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
3543530172